But there are a lot of professional software devs with zero real networking experience. Sure, they may understand the text book definition of TCP or they may even have seen pictures of fibre with labels like 'this is how far light travels in a nanosecond'. But would have no idea how to calculate the serialization latency of a 10G link, let alone know the duty cycle required to saturate one.
But none of that matters in the cloud (which is where twitter is stacking their jenga tower in the original blog post). And even if both Google and AWS have custom silicon (or at least FPGAs) doing hardware offload for their internal SDN encapsulation protocols at the server level and that their custom switches all support it, it doesn't matter. They hide all of that from you the customer and rarely even acknowledge it's existence.
If anything in HFT environments, people use L2 switches for latency reasons. Those operate at the Ethernet level, so they don't really care about IP at all.
Anyway I don't see why that's specific to electronic trading or even just a low-latency concern. Sending the same traffic to hundreds of people with unicast means using hundreds of times the bandwidth, which is a huge problem.